Attention has recently been given to the UWB as new means of communication. The UWB allows transmission of large-volume data by exploiting a wide range of frequencies in a distance as short as approximately 10 m. For example, according to the definition specified by the FCC (Federal Communication Commission) of the United States, the planned usable frequency band falls in a range of from 3.1 Ghz to 10.6 Ghz. That is, the OWE is characterized by using an extremely wide frequency band.
Recent years have seen active studies and researches on ultra-wideband filters applicable to such an UWB. For example, there is a report saying that a bandpass filter based on the principle of a directional coupler has succeeded in obtaining broadband characteristics of providing a pass band width which exceeds 100% in terms of fractional bandwidth (bandwidth/center frequency) (for example, refer to the nonpatent literature “Ultra-wideband Bandpass Filters Using Microstrip-CPW Broadside-Coupled Structure” excerpted from the collection of conference papers dated March, 2005 (C-2-114 p. 147) published by the Institute of Electronics, Information and Communication Engineers).
Meanwhile, as a commonly-used conventional filter, there is known a bandpass filter constructed by coupling together a plurality of juxtaposed quarter-wavelength strip line resonators (for example, refer to Japanese Unexamined Patent Publication JP-A 2004-180032).
However, both of the bandpass filter proposed in the nonpatent literature and the bandpass filter proposed in JP-A 2004-180032 pose some problems and are thus unsuitable for use as a bandpass filter for the UWB.
For example, a problem encountered in the bandpass filter proposed in the nonpatent literature is that the pass band width is too wide. More specifically, the UWB basically utilizes frequency bands ranging from 3.1 Ghz to 10.6 Ghz, and this has led the ITU-R (International Telecommunication Union Radiocommunications Sector) to devise a plan to divide the UWB bandwidth into “Low Band” using bandwidths ranging from 3.1 Ghz to approximately 4.7 Ghz and “High Band” using bandwidths ranging from approximately 6 Ghz to 10.6 Ghz in order to avoid the use of “5.3 Ghz” adopted in the IEEE 802.11.a standard. Hence, in a filter for use in each of the Low Band and the High Band of the UWB, it is required that there be both a pass band width of approximately 40% to 50% in terms of fractional bandwidth and occurrence of attenuation at 5.3 GHz. After all, the bandpass filter proposed in the nonpatent literature 1 having the characteristics of offering a pass band width which exceeds 100% in terms of fractional bandwidth is too wide in pass band width to be suitably used.
On the other hand, a conventional quarter-wavelength resonator-using bandpass filter has a too narrow pass band width. Even in the bandpass filter disclosed in JP-A 2004-180032 that has been devised to achieve widening of bandwidth, the pass band width is less than 10% in terms of fractional bandwidth. After all, this bandpass filter is also unsuitable for use as a UWB-adaptive bandpass filter which is required to offer a pass band width as wide as 40% to 50% in terms of fractional bandwidth.